The present invention relates to the field of power supply circuit arrangements. More particularly, the present invention relates to the field of power factor correction (PFC) circuit arrangements.
An off-line power converter is one in which power is derived from an alternating-current (AC) power source (e.g., 120 volts AC). In such a converter, a power factor correction (PFC) technique may be used to maintain the current drawn from the AC source in phase with the AC voltage so that that the converter appears as a resistive load to the AC source. This is important for non-linear loads in order to improve the power factor and to reduce harmonics, which might otherwise be introduced to the AC source.
A typical PFC technique involves use of a switching mode power converter in which the input current is switched at a frequency that is higher than the AC line frequency and modulated so as to follow the AC voltage signal in time and amplitude proportionately. In addition, the output voltage level is typically monitored, compared to a predetermined desired level and a response is developed to more precisely attain the desired output voltage. Conventional modulation techniques include pulse-width modulation (PWM) and frequency modulation.
Efficient, economical and effective techniques for power factor correction are increasingly sought after. It is to these ends that present invention is directed.
The present invention relates to a power factor correction circuit arrangement. A rectified alternating-current (AC) input signal may be applied across inputs of a voltage converter, such as a boost converter. Current drawn by the voltage converter may be sensed to form a first sensing signal that is representative of the current. The rectified input voltage may be converted to a second sensing signal that is representative of the AC input signal. Switching in the power converter may be adjusted in a first feedback loop to equalize the first and second sensing signals and, thus, the current drawn is regulated to remain in phase with the AC input signal. A second feedback loop adjusts switching so as to regulate the output voltage level of the voltage converter and, thus, controls power delivered to the load.
In accordance with an aspect of the invention, a series-coupled resistor pair may be coupled across the inputs of the voltage converter circuit. A first input of an amplifier, such as an operational amplifier or a transconductance amplifier, may be coupled to an intermediate node of the series coupled pair, while a second input of the amplifier may be coupled to a reference voltage level (e.g., ground). Thus, the intermediate node may be held to the reference voltage level by the amplifier. Current applied to the amplifier from the intermediate node may be representative of a difference between the first and second sensing signals. Accordingly, the current applied to the amplifier may be representative of a difference between the AC input signal and the current sensing signal. An output of the amplifier may control a duty cycle of a main power switch so as to maintain the sensing signal in phase with the AC input signal. A modulation circuit coupled to the output of the voltage converter may control the amount of power delivered to a load by adjusting one of the resistors of series-coupled pair in a feedback loop.
The invention is effective to ensure that the input current follows the AC line voltage in time and amplitude proportionately and also to provide a regulated output voltage.